Ultrastructure of HRP-identified sympathetic preganglionic neurons in cats

Electron microscopic studies were conducted on cat sympathetic preganglionic neurons which were labeled using retrograde transport of horseradish peroxidase from the injection site in the stellate ganglion to somas in the thoracic spinal cord. Based on size, the identified somas were divided into two groups: large multipolar somas (25-35 micron), usually having three dendrites with a mean diameter of 8 micron, and small fusiform somas (12-24 micron), with two primary dendrites with a mean diameter of about 4 micron. The ultrastructures of the perikarya and dendrites were similar to other spinal neurons. Three types of terminals were identified: 1) round asymmetric terminals, 2) pleomorphic symmetric terminals, and 3) round symmetric terminals. On the soma, pleomorphic terminals were more numerous than round asymmetric terminals. On the dendrites, the density of round asymmetric terminals was similar to pleomorphic symmetric terminals. Round symmetric terminals represent only a small percent on either the soma or dendrites. Because of the greater total density of terminals on the dendrites than soma, pleomorphic symmetric and round asymmetric terminals are primarily located on the dendrites. Very few dense-core vesicles characteristic of monomamine terminals were observed, perhaps due to the fixative used.